Through the in-situ synthesizing the doping of zirconium dioxide and graphite, the ZrC particles reinforced NiCrBSi composite coating was produced onto the mild steel by laser cladding. Microstructure, phase constitution of NiCrBSi+Zr composite coating were characterized by scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and X-ray diffractometer(XRD). And the morphology, formation and growth mechanism and distribution of ZrC reinforcements were studied and discussed, and the hardness of coating was investigated as well. The results showed that ZrC reinforcements were uniformly distributed in the upper and middle coating, only little was in the bottom of coating. The morphology of ZrC were mainly consist of triangle, quadrilateral similar to rhombus and long-bar shape, which contributes to obstruction from the interdendritic growth, the collision and sintering caused by high convection and repulsion between solid-liquid interface in the molten pool. Hardness of composite coating was improved by ZrC reinforcements and carbide such as M₂₃C_6, which contribute to its high hardness. Finally, the hardness of composite coating can reach to HV_0.2 720.